Platens are used in various printing machines to support a print medium (e.g., paper) while the printing machine produces the desired text and/or graphics on the print medium. Platens are generally made with a rigid cylindrical central shaft and a semi-rigid compliant printing layer surrounding the outer surface of the shaft. Such compliant printing surface is formed of a material, or a composition of materials, that provides sufficient friction to control the movement of the print medium thereon as the platen rotates about its longitudinal axis. In typewriting machines, the semi-rigid compliant printing layer is chosen to also provide sound deadening qualities and minimal deformation as the type forming the characters impacts the print medium. In this regard see, for example, U.S. Pat. No. 731,834 (F. F. Anderson), which issued on Jun, 23, 1903, U.S. Pat. No. 4,900,175 (H. Ikeda et al.), which issued on Feb. 13, 1990, and the article entitled "Rigid Foam Platen" by G. A. Duggins et al. in the IBM Technical Disclosure Bulletin, Vol. 17, No. 4, Sep. 1974 at page 1115.
Platens are also used in non-impact printers such as ink-jet printers. Non-impact printers are so called because their printing mechanism does not touch the paper or print medium. More particularly, ink-jet printers use electrically charged ink droplets that are sprayed between electrically charged deflection plates to direct the ink droplets and form the desired image on the print medium disposed on a platen. Thermal printers, on the other hand, typically use a specially-coated heat-sensitive print medium, such as paper, which moves between a platen and a thermal print head. The thermal print head comprises, for example, a linear array of heating elements (forming individual pixels) which contact the heat-sensitive print medium with a predetermined amount of pressure. The heating elements are then energized so as to provide a predetermined amount of heat to each pixel area. The heat from each of the energized heating elements reacts with the heat-sensitive print medium therebeneath to form a separate pixel of the desired image. The next line of pixels of the desired image are formed by advancing the platen, and the print medium thereon, by a predetermined distance passed the thermal print head. In certain heat-sensitive or thermosensitive papers, as explained in the article entitled "Mechanisms of Color Formation On thermo-sensitive Paper" by A. Igarashi et al. in the book Advances In Non-Impact Printing Technologies For Computer and Office Applications, Edited by J. Gaynor, Van Nostrand Reinhold, Company, 1982, at pages 886-892, a thermo-sensitive layer of certain components is provided on the paper. The subsequent predetermined heating of each pixel (via a heater element on a thermal print head) changes the light absorption characteristics of the thermo-sensitive layer.
In certain thermal printers, a dye receiving member is fed onto a platen and then a dye bearing web is placed in contact with the dye receiving member. As the platen rotates, the dye receiving member and the dye bearing web thereon are brought under the thermal print head. Heat from the thermal print head transfers a predetermined amount of dye from the dye bearing web to the dye receiving member. The dye receiving member and dye bearing web are advanced a predetermined number of increments until a complete image layer has been deposited. In these applications, the overall image may require multiple dye layers to be deposited on the dye receiving member, such as in the creation of continuous tone sublimation dye images. The overall image quality where multiple overlapping dye layers are used is dependent on the registration of each of the dye layer to each of the other overlapping dye layers.
The article entitled "Pulse Count Modulation: A Novel Head Drive Method For Thermal Printing" by M. D. Fiscella et al. in the publication Hard Copy and Printing Technologies, Volume 1252, Proceedings of the SPIE, Feb. 13-14, 1990, Santa Clara, Ca. at pages 156-167, discusses a continuous tone thermal dye diffusion printer designed by the Eastman Kodak Company using a pulse count modulation thermal print head drive. In the printing process, a hot heater element of the thermal print head diffuses dye from a donor sheet into a dye receiving member (e.g., resin coated paper) to form a pixel of a desired image. In thermal dye diffusion printing, the amount of dye transferred to a pixel, and the optical density level of the pixel, are a function of the amount of heat produced at a given heater element and the length of time the heater element is hot.
In certain printers such as continuous tone thermal dye diffusion printers, several dye layers must be deposited to produce the desired image. The fore, after a dye layer has been deposited, the dye receiving member is returned to a starting position for each succession dye layer. It is desirable that each successive dye layer precisely overlay the preceding dye layers for optimum image quality. Because prior art platens are typically covered with an elastomer, a certain amount of mis-registration results from the rewinding operation.
Additionally, in certain printers it is desirable that the diameter of the rotating platen be as small as possible. With small diameter elastomeric coated platens, it is usually impossible to fixedly clamp the dye receiving member to the platen because the circumference of the platen is smaller than the length of the image to be produced. In a first known embodiment using small diameter elastomeric coated platens, the dye receiving member is brought to a starting position across the platen, and a dye layer is produced along the dye receiving member. For each succeeding dye layer application, the platen and the dye receiving member are counter-rotated the same degree, or amount of rotation, as was performed during printing of each prior dye layer. It is found that with the above-described counter-rotating method, the dye receiving member does not return to the exact same starting position each time. This mis-registration is due to the compliant nature of the elastomeric coating of the platen.
In a second known embodiment using small diameter platens, the dye receiving member movement is controlled by external, hard surface, capstan drive print rollers that reduce the mis-registration found in platen rewind printers. With the additional capstan print rollers, the overall printing mechanism is necessarily more complex and expensive. Additionally, such print mechanism produced a large non-printed area on the dye receiving member, which area is at least equivalent to the distance between the printing "nip" (where the dye receiving member engages the platen and the thermal print head) and the capstan "nip" (where the dye receiving member engages the external capstan roller).
With most printers, pinch rollers are used at one or more areas around a platen to provide a force against the print medium and, in turn, the compliant material of the platen during the printing process. However, at the conclusion of the printing process, if the pinch roller remains in forced contact with the compliant platen, the platen takes a permanent set, or dent, in that area. In subsequent printing operations the image produced on the print medium experiences a perceptible loss of density in the area of the dent. Therefore, to avoid such loss of density, prior art printers include mechanisms which pull the pinch rollers from the platen during non-printing periods to avoid producing dents in the platen.
It is desirable to have a simple and inexpensive printer which provides a good quality of registration while avoiding the need for capstan roller mechanisms, and which prevents the production of dents in the platen.